BUG: List of years (as string) raises UserWarning with to_datetime

[jrmylow]

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Reproducible Example

>>> pandas.to_datetime(["2019"])
DatetimeIndex(['2019-01-01'], dtype='datetime64[ns]', freq=None)
>>> pandas.to_datetime(["2030-01-01", "2020-01-01"]) 
DatetimeIndex(['2030-01-01', '2020-01-01'], dtype='datetime64[ns]', freq=None)

>>> pandas.to_datetime(["2019", "2020"]) 
<stdin>:1: UserWarning: Could not infer format, so each element will be parsed individually, falling back to `dateutil`. To ensure parsing is consistent and as-expected, please specify a format.
DatetimeIndex(['2019-01-01', '2020-01-01'], dtype='datetime64[ns]', freq=None)

Issue Description

List of years (as strings) provided to to_datetime raises UserWarning. For some reason, single years and lists of dates do not trigger this warning. This is breaking tests in CI.

Expected Behavior

Expect no userwarning.

Installed Versions

N/A

[jrmylow]

Update: The calling chain is:

• to_datetime is called from tests/series/methods/test_nlargest.py
• Within core/tools/datetimes.py it calls:
  • to_datetime
  • convert_listlike
  • _convert_listlike_datetimes
  • _guess_datetime_format_for_array
  • guess_datetime_format which returns None
• Over in _libs/tslibs/parsing.pyx:
  • guess_datetime_format
  • dateutil_parse which raises ValueError
  • default.replace(**repl) raises DateParseError

When testing with python:

import datetime as dt
a = dt.datetime.today()
a.replace(**{'year': 2003})

# alternatively
a.replace(year=2003)

Both will raise ValueError: day is out of range for month.

I suspect this may be a weird change in cpython? Let me check there

[jrmylow]

Update: "year is out of day for month" is raised because today is Feb 29th.

[MarcoGorelli]

For some reason, single years and lists of dates do not trigger this warning.

if there's only a single element, then there's nothing to be inconsistent with

I think this is expected

Update: "year is out of day for month" is raised because today is Feb 29th.

yup, this is the issue, thanks for the triage :)